It all depends on how thick your work is. If your piece is thin, then you may need to wait overnight to apply the other glaze because the pot will be saturated from glazing the inside.   When double dipping, the first glaze should not be totally dry or it's likely to come off the pot when you do the second dip. Dry to the touch is usually sufficient, although some glazes need to be drier than others. You just need to get to know your glazes well and figure out what works best.